Experimental Study On Upstream Development Of Piping Channel In Levee Foundation

Piping is a main kind of seepage failure, which threatens safety of levee. The mechanism and factors are complicated. At present the laboratory experiment and numerical simulation study pay more attention to process of piping occurrence, but less on process of piping upstream development. According to the problem above, this paper makes the following work:(1) Based on experience of former experiment, make wide-type sand tank model. Explore the solution and technique of modeling overburden, modeling exit of piping, setting reserved piping channel and avoiding boundary effect, in order to guarantee the result. Transform the narrow-type sand tank model to improve precision of parameters, for clearer law of experiment.(2) Wide-type sand tank experiment observes the process of upstream movement of piping channel in natural state, in working condition of rigid overburden and flexible overburden. Result shows that slender channel occurs in contact surface under rigid overburden, which develops upstream tortuously. Channel forking in tip is a general development form; wide channel occurs in contact surface under flexible overburden, which develops uniformly and slowly. Piping channel moves upstream intermittently in two kinds of working condition. Classify and discuss the phenomenon of middle island, main and branch channel, alternate scour and uniform scour.(3) Narrow-type sand tank experiment appears a same law as wide-type experiment. Result shows that tip hydraulic gradient decreases after upstream movement, which makes channel steady. There exists critical tip gradient. Channel develops upstream when tip gradient is larger than critical value and becomes steady when it’s below the critical value. Critical tip gradient relates to tip slope angle, the smaller slope angle with larger critical gradient. Tip hydraulic gradient is better than average hydraulic gradient in judging condition of channel upstream movement.(4) Via the data of narrow-type experiment, explain the process of intermittent upstream movement, scour of middle island and channel expansion in wide-type experiment.